Due to its characteristics of being easily applicable to various products and electrical characteristics such as a high energy density, a secondary battery is not only commonly applied to a portable equipment, but universally applied to an electric vehicle (EV), a hybrid vehicle (HV), or an energy storage system that is propelled by an electric motor. This secondary battery is gaining attention for its primary advantage of remarkably reducing the use of fossil fuels and not generating by-products from the use of energy, making it a new eco-friendly and energy efficient source of energy.
A secondary battery can be charged and discharged repeatedly by an electrochemical reaction between elements including a cathode current collector, an anode current collector, a separator, an active material, an electrolyte solution, and the like. By way of example, a widely used lithium polymer secondary battery has an operating voltage in a range of about 3.7V to about 4.2V. Accordingly, to obtain a high output battery pack for use in an electric vehicle, a plurality of unit secondary battery cells are connected in series to construct a battery pack.
In addition to this basic structure, the battery pack further includes a battery management system (BMS) to monitor and control a state of a secondary battery by applying an algorithm for control of power supply to a driving load such as a motor or the like, measurement of electrical characteristics such as current or voltage, charge/discharge control, voltage equalization control, state of charge (SOC) estimation, and the like.
Recently, with the increasing need for a high-capacity structure as well as utilization as an energy storage source, the demand for a battery pack of a multi-module structure in which a plurality of battery modules including a plurality of secondary battery cells are assembled, is also increasing.
Because the battery pack of the multi-module structure includes a plurality of secondary battery cells, there is a limitation in controlling the charge/discharge state of all the secondary battery cells using a single BMS. Accordingly, a recent technology has been introduced in which a BMS is provided to each battery module included in the battery pack, the BMSs are designated as a slave BMS, and an additional BMS is provided as a master BMS to control the slave BMSs, such that the charge and discharge of each battery module is controlled in a master-slave mode. The slave BMS stands by in a sleep mode during a normal state, and wakes up by a control signal from the master BMS.
FIG. 1 is a schematic circuit diagram illustrating a wakeup apparatus 13 to transmit a wakeup signal outputted from a master BMS 11 to a slave BMS 12 according to a conventional art.
Referring to FIG. 1, a plurality of battery modules 14 are connected in series to construct a battery pack 15, and each battery module 14 is connected to the slave BMS 12. Also, an opto-coupler as the wakeup apparatus 13 is connected to transmit a wakeup signal outputted from the master BMS 11 to each slave BMS 12.
An opto-coupler refers to a switching device including a light emitting source (input) and an optical detector (output). Generally, an infrared light emitting diode (LED) is used as the light emitting source, and a photodiode or a phototransistor that turns on by light is used as the optical detector. Thus, when current flows to the input side, the light emitting source emits light and the photodiode or phototransistor at the output side is turned on. That is, the switching device is turned on or off by light, rather than by electrical coupling.
The input side of the opto-coupler 13 is connected to the master BMS 11, and the output side is connected to the slave BMS 12. Thus, when the master BMS 11 outputs a wakeup signal, the wakeup signal is transmitted to the slave BMS 12 via the opto-coupler 13.
The use of the opto-coupler 13 to connect the master BMS 11 and the slave BMS 12 provides an advantage that the master BMS 11 and the slave BMS 12 are electrically isolated. As a result, at the same time of transmitting a wake up signal, a reverse current in which a high voltage current flowing from the battery pack 15 is input to the side of the master BMS 11 may be prevented, and an electromagnetic wave generated during charging/discharging of the battery pack 15 may be less influenced.
However, to enable the slave BMS 12 to wake up while maintaining the insulation between the master BMS 11 and the slave BMS 12, it requires as many opto-couplers 13 as there are slave BMSs in the battery pack 15 as shown in FIG. 1. Further, the opto-coupler 13 is a semiconductor device which means its cost is somewhat high, and when a plurality of the opto-couplers 13 is used, the total cost of the BMS or battery pack may increase. Accordingly, there is a need for research in waking up the slave BMS 12 while maintaining the insulation between the master BMS 11 and the slave BMS 12.